1. Field of the Invention
The present invention relates generally to nuclear reactor coolant system pumps and, more particularly, is concerned with a converging spout discharge nozzle on an offset pump casing.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and reactor coolant pumps.
In one version of the reactor coolant system used in a nuclear power plant, the reactor coolant pumps are high inertia pumps hermetically sealed and mounted to the one steam generator in the respective coolant loop. Each pump has an outer casing, a central axially extending rotor rotatably mounted at its opposite ends by upper and lower bearings, and a canned motor located about the pump rotor between the upper and lower bearings. The motor includes a rotor section mounted for rotation on the pump rotor and a stator stationarily mounted to the casing about the rotor section.
Also, each pump includes an impeller mounted at one end of the pump rotor. The pump impeller rotates with the rotor, drawing reactor coolant water axially through a central inlet nozzle in the pump casing. The impeller also discharges the water tangentially through a peripheral outlet nozzle in the pump casing.
Typically, pump casing geometry is of the volute type in which the outlet nozzle is disposed in the plane of rotation of the impeller so that the path of the water flow from the impeller is directly to the outlet nozzle. However, volute casings have large diameters and are heavy and costly. In order to reduce pump space, cost, and weight, an offset type casing can be employed. In an offset casing, the output nozzle is offset axially from the plane of rotation of the impeller allowing a smaller casing diameter but requiring that the water must now flow along an axially-directed helical path before reaching the outlet nozzle.
A penalty in efficiency thus results when substituting an offset casing for a volute type, since the offset casing flow area is usually larger and not matched correctly with the area required by the fluid flow. This efficiency penalty is caused by dumping (mixing) losses in the main casing and flow contraction losses in the outlet nozzle.
Consequently, a need exists for improvement of the offset pump casing design so as to reduce the efficiency penalty associated therewith.